THE FLORIDA REEFS. 91 



for granted that the most favorable conditions for their support 

 exist ; and this condition we assume to be an abundance of food, 

 brought to them by the great oceanic currents passing over the 

 regions where these submarine plateaux are forming. We know 

 as yet too little of the fauna of the oceanic basins to be able to 

 affirm how far the population of the bottom depends upon the 

 food it receives from oceanic currents. We can only judge by 

 analogy. No marine fauna has been explored which equals 

 in variety or in the number of its individuals that of the Ca- 

 ribbean and of the Gulf of Mexico, from the depth of two 

 hundred and fifty to about one thousand fathoms. It has 

 proved richest in the districts most favorably situated with re- 

 gard to the currents and the food supply they bring in their 

 track. It is but natural to extend this effect to other oce- 

 anic currents, and in their track we may therefore expect to 

 find the most favorable conditions for the support of an im- 

 mense fauna. In fact, the question of food is of the utmost 

 importance to the distribution, not only of marine, but of ter- 

 restrial animals ; and the absence or presence of an abundant 

 supply of suitable nourishment must of necessity be an all- 

 important factor in the character and variety of the fauna of 

 any place or period, — far more influential, perhaps, than the 

 many obscure physical causes upon which we are so apt to 

 explain the distribution of animal life. On the continental 

 ledges, where the shore detritus is gradually accumulated, brino- 

 ing with it a large amount of animal and vegetable food, we 

 find the most populous fauna near the hundred-fathom line. 

 When, in addition to the action of the influences which have 

 accumulated the shore detritus, we have a continental shore or 

 plateau bathed by a great and powerful current, bringing with 

 it an abundance of pelagic life, we may expect a superabundant 

 supply of food, and consequently a fauna of unusual richness 

 and variety. The fauna of the Pourtales Plateau, of the hun- 

 dred-fathom slope to the westward of the Tortugas, of the 

 northeastern slope of the Yucatan plateau, of the windward side 

 of the Lesser Antilles, and of the continental slope of the eastern 

 coast of the United States below the hundred-fathom line, are 

 all examples of such districts supporting a marine fauna of sur- 



